THM250 - Used in small 4 cylinder cars (Vega, Sunbird, Monza)
from 1973 to 1981 and some Nova/Camaro/Chevelle 6 cylinders in 1974 &
75. The 4 cylinder models have 4 large cooling holes near the torque converter,
while on the "bigger cars" there are no cooling holes.

THM200-R4 - Used in GM rear wheel drive cars with the 3.8 V6,
305 & 350 Olds and 301 pontiac from 1981 to 1989. This is a 4 speed,
overdrive unit.

THM350 - Medium duty 3 speed trans used from 1969 to 1989, Used
in Buick, Olds, Pontiac and Chevrolet cars and Chevy & GMC light trucks
until 1987. Generally used in straight and V6 and small block engines.
Generally able to handle up to 400 foot pounds of torque.

THM375B - This trans is identical to the THM350, but is considered
a heavier duty transmission.

THM375 - This transmission is a derivative of the THM400 sharing
the external appearance of the 400. It'll be marked 375-THM on the bottom
of the tailshaft housing. It's a light duty version of the THM400, has
ABSOLUTELY NO RELATION to the THM375B, and was found in 1972 to 1976 full-size
cars. The Turbo 375 was modified to accept the Turbo 350 driveshaft yoke.

THM400 - The THM400 was GM's Heavy-duty 3 speed transmission used
from 1964 to 1990. Used on large displacement, high torque engines and
engines with towing packages. Generally found in Chevy & GM trucks,
cadilacs and large displacement engined, full size GM rear wheel drive
cars.

THM700-R4 - 4 Speed transmission used in GM cars and light trucks
with the 2.8, 3.1, 4.3, V6, Chevrolet built 305, 350 and 454 engines from
1982 to 1992.

3L80 - The 3L80 is a renamed THM400, used in the early 1990's.
The 3L80HD would be the THM475 (heavy duty unit).

4L80 - An overdrive model of the THM400. The 4L80E would be the
"electronic" model.

Transmissions with a suffix of "C" on the end of the type (THM200C) indicate
the transmission has a lock up torque converter.

Automatic Transmission Identification & Decoding

Specific methods of identification are as follows (and yes, this section
needs to be rewritten).... Some transmissions have a code stamped onto
them. Others have a tag rivited to the case.

Most transmissions will have a "source serial number" (chassis vin) usually
found close to the transmission code. This number will contain division
ID #, model year, assembly plant and production sequence (last 6 digits)
of the VIN stamped onto the transmission. Example: 19N500001. The
source serial number is from the CAR the transmission originally went
into, NOT the assembly plant.

Location of source serial number

On the THM350, the VIN will be stamped on either the drivers side housing
near the shifter, on the right side of the housing just above the pan,
or on a boss behind the bellhousing flange on the passenger side of the
transmission.

On the THM400 the VIN is stamped on a machined surface just above the
pan on the drivers side.

On the THM200-R4 the VIN is stamped onto the housing towards the rear
of the pan, on the drivers side.

On the THM700-R4 the VIN is either stamped onto a boss behind the bellhousing
flange on the passenger side, just above the pan on the drivers side,
or just behind the governor cover, stamped vertically where the case meets
the tailshaft housing.

Prior to 1967, transmission ID numbers contained the plant prefix
code, month and date of production (expressed numerically) and a shift
code (D = Day, N = Night). From 1967 on, the ID number contained the transmission
type or plant prefix, Date (coded below) and a shift code. The constants
in decoding the trans ID number are the date the transmission was produced.

Manual Transmission Identification & Description

Manual Transmission Eyeballing

Muncie (left) Saginaw
(center) Borg Warner (right)

To quickly finger what you have, notice in the above picture:
The muncie reverse lever is in the extention housing, and the cover has
7 bolts.
The Saginaw reverse shift lever is on the cover, and the cover has 7 bolts.
The Borg Warner has a 9 bolt cover.
Muncies are the hot tip, since they usually were used with high performance
engines. Again on the muncie, and looking at the cover, 3rd/4th gear lever
is on the left. 1st/2nd gear lever is on the right.

Further Muncie identification
3 Muncie 4 speeds were offered. The M-20 wide ratio, the M-21 close ratio,
and the "Extra heavy duty close ratio" M-22, also known as the "Rock Crusher".
The M-22 was generally installed behind severely powerful (high torque)
big-block engines. The M-22 is distinguished from an M-21 by the angle
of the gear teeth. The M-22 has "straight cut" gears. Due to this, the
box tends to be noiser (whines or howls almost like a blower, which is
why it's called a "rock crusher") than other boxes. The M-21 has a higher
pitch angle on its gear teeth.

The close ratio gearboxs generally came with 3.73+ gear ratio'ed cars.
3.55 geared and higher (numerically lower) cars used the wide ratio gearbox.

Due to the wide combination of id's, stampings, numbers, etc... varied
and even GM couldn't keep track of what was what, it's best to ID your
gearbox visually, taking what is stamped on the box FWIW. The best way
to ID the transmission is to count teeth.